This invention relates to apparatus for vibrating or otherwise moving an article in limited displacement with multiple degrees of freedom.
Power plant equipment that may be subjected to seismic environments, space hardware, and a variety of other devices that may have to withstand shock or vibration, are often tested in an apparatus that produces dynamic loads simultaneously in several degrees of freedom, i.e., with motion along and/or rotation about, up to three perpendicular axes. Normally, such vibration is produced by hydraulic actuators which are attached at one end to a reaction mass and at another end to a table upon which the specimen-to-be-tested is mounted. The hydraulic actuators typically have cylinders and pistons respectively pivotally mounted to the reaction mass and table, to accommodate motion of the table perpendicular to the direction of movement of the piston of each hydraulic actuator. Such a mounting typically results in loading at the pivoting joints which can lead to fatigue failure, and results in lateral loads on the guide bearings of the piston rod as well as bending stresses on the piston itself. In addition, there is typically a long and sometimes narrow load path between the pressurized face of the piston and the table, which results in lower stiffness than desirable. Also, considerable space is required for the reaction masses on which the actuators are supported.